The prosthesis may involve a replacement foot, ankle, and foreleg, or the latter plus a knee and a preselected length of the upper leg or femur; depending upon the needs of the particular patient/individual and the nature of the affliction requiring the prosthesis. The prosthesis may even include a hip in an extreme case.
The functional component of a prosthesis is composed of an endoskeletal system composed of metal (load bearing); elements, and usually motion imparting or permitting joints, pivots, bearings permitting turning or partial turning and flexing or partial flexing; the latter for the purpose of simulating a natural movement or gait. The endoskeletal component may also include certain rigid plastics, where their individual strengths and lightness permit their substitution for traditionally used metals of light weight, such as aluminum or titanium etc. A further important component of a prosthesis is a surrounding element or elements which mask or hide the endoskeletal component and additionally serve to simulate the appearance and contour of the missing limb.
One technique for hiding, covering, shielding or masking the structural (and/or endoskeletal) component contemplates an encapsulation of the structural components with a matrix substance formed from a liquid resin containing a blowing agent which by/through application and control of appropriate conditions creates a foamed-in-place matrix, mass or body, which is thereafter processed by appropriate tooling and finishing to approximate the appearance/contour of the limb replaced. In a variant approach, a form or enclosure is positioned about the endoskeletal region as to define a cavity; and thence, there is introduced thereto, a volume of liquid resin containing a suitable blowing agent. Thereafter heat and/or pressure is applied to convert same to a cellular matrix which is hopefully of lower weight.
The product of the techniques as described above requires considerable time, handicraft and tooling in order to attempt to tailor make the desired cover or shield; and at the same time simulating the contour/appearance of an actual limb. Usually the resulting product is less than satisfactory because it lacks durability, is heavy, compared to the cover of the invention, and requires much more labor to install.
The foregoing also has the drawback that the foam-in-place material must be usually destroyed if the structural component, which may include hydraulic units, control units sockets etc. and possibly other elements requiring inspection, requires lubrication and/or service in any respect.
Vinyl resins such as Geon 121 manufactured by B. F. Goodrich Co. of Akron, Ohio are used in making a reshapeable prosthesis, as disclosed in U.S. Pat. No. 4,704,129. The technique of the latter patent commences with the formation of a cone from the vinyl resin ("Geon 121") via a slush cast technique employed in a heat transfer mold. The technique further employs a model about which the cone is located for reforming and resetting same to the desired configuration usually preceded by a preheating step. The final product, as described in the '129 patent, is a socket separated from the mold by the application of compressed air thru appropriately provided ports.
It is also presently conventional to prepare covers which are cellular and shaped by the expedient/technique of injection molding. In this approach a multi-cavity mold, usually fabricated of stainless steel, usually two mating plates machined to contain there between, a plurality of cavities or voids which define an elongate, open ended structure with the outer surface having the appearance of the limb in question. A liquid resin containing a suitable blowing agent is pumped into each of the voids of the multi cavity mold, which is then heated frequently thru the circulation of steam in and through internal passageways formed in the body of the mold, whereupon the resin expands due to the heat activation of the blowing agent to yield the molded (cured) product which is cellular. The piece may be entirely cylindrical, as achieved by the use of mold inserts and appropriate mold design, or the product may be a half cover which is combined with a second matching half cover and secured by suitable adhesive at the mating edges to yield a cover surrounding the endoskeletal system or structure.
While considerable uniformity and some economy is achieved by the injection molding technique; the end product has a rather rubberlike composition and is also amorphous and is quite difficult to cut and trim in order to custom fit it with complementary components of the prosthesis, for example at the ankle or at the knee or just above the knee, where the limb in question is the leg. Also use of abrasion tools; such as grinders or sanders, on the molded cover results in a very rough and cosmetically displeasing surface.